The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used to sense externally applied magnetic fields.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as field sensors. Magnetometers and other magnetic field sensing devices are used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic field sensed thereby in a variety of situations.
Such sensors can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic conditions therein. These devices may be provided on a surface of a monolithic integrated circuit chip to provide convenient electrical connections between the device and the operating circuitry therefor in the integrated circuit chip. Otherwise, they may be mounted on another structure conveniently close to the sensor for this purpose.
Ferromagnetic thin-film sensors can be made very small when so constructed. Such sensors are often provided in the form of an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic thin-film is provided. In such "sandwich" structures, reducing the thickness of the ferromagnetic thin-films in the intermediate layer has been shown to lead to a "giant magnetoresistive effect" being present. This effect can be enhanced by having additional alternating ones of such films and layers, i.e. superlattices. This effect can yield a magnetoresistive response which can be in the range of up to an order of magnitude greater than that due to the well-known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response in ferromagnetic thin-films, varying differences between the direction of the magnetization vector in such a thin-film and the direction of a sensing current passed through that film in turn lead to varying differences in the effective electrical resistance of the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the film and the current direction are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance of such a magnetoresistive ferromagnetic thin-film exhibiting this response can be shown to be given by a constant value, representing the minimum resistance present, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance follows a square of the cosine of that angle.
As a result, external magnetic fields supplied for operating a film sensor of this sort can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film portion. This axis exists in the film because of an anisotropy present therein typically resulting from depositing the film in the presence of an externally supplied magnetic field during deposition of the film that is oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of a sensing device using this resulting film, such externally supplied magnetic fields for operating the film sensor can vary the magnetization vector angle to such an extent as to cause switching of that film's magnetization vector between two stable states which occur as magnetizations oriented in opposite directions along the established easy axis. The state of the magnetization vector in such a film portion can be measured, or sensed, by the change in resistance encountered by a current directed through this film portion.
In contrast to this arrangement, resistance in the plane of either of the ferromagnetic thin-films in the "sandwich" structure is isotropic with respect to the giant magnetoresistive effect rather than depending on the direction of a sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect has a magnetization dependent component to resistance that varies as the cosine of the angle between the magnetizations in the two ferromagnetic thin-films on either side of the intermediate layer. In the giant magnetoresistive effect, the electrical resistance through the "sandwich" or superlattice is lower if the magnetizations in the two separated ferromagnetic thin-films are parallel than it is if these magnetizations are antiparallel, i.e. oriented in opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas very thin films are a fundamental requirement to obtain a significant giant magnetoresistive effect. The total electrical resistance in such a magnetoresistive ferromagnetic thin-film "sandwich" structure can be shown again to be given by a constant value, representing the minimum resistance present, plus an additional value depending on the angle between the magnetization vectors and the two films as indicated above.
One common magnetic field sensing situation is the sensing of magnetization changes along a data recording track selected from many such tracks in the magnetic media of a magnetic data storage system. As these tracks are made narrower and narrower to permit increases in the data density in the magnetic media, inductive sensing of the magnetization changes along any of those tracks becomes less feasible. The smaller magnetization volumes lead to smaller outputs from an inductive sensor, and there is a limit to the number of turns in the coil used in such a sensor which can be provided to increase the output signal. Even in thin-film versions thereof, such inductive sensing structures remain relatively thick which becomes a problem as the tracks are made more narrow. Thus, sensing of the magnetization changes along the track using thin-film magnetoresistive sensors has become attractive.
Such magnetoresistive sensors for detecting magnetization changes along a track in the magnetic media are typically formed with the magnetoresistive sensor film in a rectangular shape, and sensors based on such films in initial designs therefor had such a sensing film positioned between a pair of highly permeable magnetic material shielding poles with a long side of the film's rectangular shape located adjacent the magnetic media to result in what is oftentimes termed a horizontal sensor. More recently, such magnetoresistive sensors have had an alternative construction with such sensing films positioned between the poles with the short side of the rectangle adjacent the magnetic media to form what is often termed a vertical sensor or an "end-on" sensor. These kinds of sensors were both initially based on use of the anisotropic magnetoresistive effect in the sensing films. This effect gives a maximum change in magnetoresistance due to the sensed magnetic fields on the order of 2.5% at room temperature.
As data tracks in the magnetic media grow ever thinner coupled with use of higher densities of magnetization direction changes therealong, the need for a more efficient converter of such magnetization changes in the magnetic medium into a sufficiently large current or voltage output signal becomes greater. Hence, horizontal and vertical magnetoresistive sensors based on the "giant magnetoresistive effect" were introduced because of the greater changes in resistance possible from corresponding changes in externally applied magnetic fields. A vertical or end-on magnetoresistive sensor based on the "giant magnetoresistive effect" is typically formed with a nonmagnetic intermediate conductive metal layer having ferromagnetic layers on opposite sides of the major surfaces thereof with all layers in corresponding rectangular shapes. As before, such a vertical sensor is mounted typically between a pair of ferromagnetic material shielding poles in a narrow gap provided therebetween so that a short side edge of the rectangular film sensor is positioned adjacent the magnetic media approximately in a plane with the sides of the poles also being positioned adjacent the magnetic media with the resultant surface in this plane forming the air bearing surface. Thus, the long sides of the sensor extend inward into the gap between the poles and away from the magnetic media.
The magnitudes of the changes in voltage across the length of the vertical sensing "sandwich" structure due to magnetization transitions in the magnetic media passing thereby, leading to voltage pulses that combine to form the sensor output or "read" signal, will be determined by a) the magnitude of the sense current provided therethrough along this length which in turn results in a sense field, H.sub.sn, across the width of the sensing films, and b) the magnitudes of the angular changes in the directions of the magnetizations of the two ferromagnetic layers for corresponding changes in the applied external field due to changing magnetization directions in the magnetic media passing by the sensor, i.e. the signal field or H.sub.sg. This signal field is directed to a significant extent along the length of the vertical sensing "sandwich" because of the permeabilities and the geometries of the magnetic materials present.
The increases in magnetic flux along the length of the sensing structure, due to the corresponding rotating magnetizations of the ferromagnetic layers in response to changing applied external fields because of magnetization transistions in the magnetic media passing thereby, is proportional to the total film magnetization multiplied by 47.pi. and further multiplied by the sine of the angle between the direction of the total magnetization and an axis parallel to the surface of the magnetic media across the width of the sensing films termed the "x" axis. Hence, the degree of angular change in the directions of the magnetizations of the ferromagnetic layers in response to an externally applied field can be characterized as an effective permeability in the sensing structure in view of its relation to the increase in lengthwise flux in response to an externally applied field. This effective permeability characterizes the sensing film in the sensing structure, and, as indicated, is proportional to the sine of the just described angle which can be designated .theta..
An approximate analysis for determining the value of .theta. can be found from thermodynamic considerations through finding the total magnetic energy present in such a vertical sensor and differentiating the same with respect to .theta. followed by setting the result equal to zero to determine the value of .theta. yielding a minimum for that energy. This is equivalent to setting equal to zero the magnetic torque on the magnetization vectors in the ferromagnetic films or layers which can be written directly assuming that the magnetization of each of the ferromagnetic layers is in the plane of the layer due to the thinness of such layers as films, and assuming that the rotations of the magnetization in each of those layers are equal in magnitude but opposite in direction because of symmetry between the layers. Further assumed is that, in each of the ferromagnetic layers, the magnetizations at the long sides of those layers in the vertical sensing films are pinned parallel to those sides due to demagnetization considerations. In these circumstances, the magnetization effect of one ferromagnetic layer on the other can be found from a line integral of the total magnetic field around the sensing "sandwich" structure intermediate layer, i.e. primarily across the width of the sensor, so that the magnetic torque equation can be written as ##EQU1## Here, the quantity A is the exchange constant, M.sub.s is the saturation magnetization, and .theta. is the angle of the film magnetization away from the fabricated easy axis which is across the width of that sensing film in the direction of the x axis taken as having its zero datum at the center of the sensor width.
The factor S' is the effective separation of the two ferromagnetic layers in the sensing structure "sandwich". The effective separation is somewhat greater than the actual physical separation of those layers, i.e. the thickness of the intermediate layer, because of the turning of the magnetic flux in one ferromagnetic layer toward the other at the long edges of the vertical sensor so as to have that field pass through the separation to the other. The effective separation is determined from ##EQU2## In addition, H.sub.sg is the externally applied signal field due to the changing magnetizations in the magnetic media passing by the sensor and H.sub.sn is the sense field due to the sense current as indicated above. H.sub.b is the bias field generated by the provision of a bias current in a current strap near the sensing structure that is used to operate the vertical sensor in a more linear portion of its characteristic. H.sub.k is the anisotropy field.
The first term on the right-hand side of the equal sign in the magnetic torque equation is an exchange term representing the resulting torque on the rotating magnetization. The second term is the torque due to the signal field generated by the changing magnetizations in the magnetic media moving past the vertical sensing structure, and the torque due to the bias field provided by a current in the bias current strap. The third term is the torque due to demagnetizing effects of the fringing fields between the ferromagnetic layers. The fourth term is the torque due to the sense field resulting from the sense current passed through the vertical sensing structure in a direction parallel to the long sides thereof. Finally, the last term is the torque due to the anisotropy described above.
This anisotropy field has a value with respect to an effective easy axis which can be arranged during fabrication of the ferromagnetic films to occur along one of many possible directions in these films. As a result, the algebraic sign of the term in the above equation containing this factor will vary depending on where this axis is located in the films actually made, and will not always be negative as shown in the above torque equation which is appropriate, as an example, for the easy axis extending across the width of the vertical sensing structure.
Some easy axis directions in the ferromagnetic films have been found to be better than others in providing a larger output signal for a given externally applied field input. One such arrangement is effectively having the easy axis extend along the length of the vertical sensing structure even though the ferromagnetic films were fabricated with an externally applied field directed along the width of the structure which would typically result in the easy axis being along the structure width or "x" axis. This moving of the effective easy axis from across the width of the vertical sensing structure to along its length comes about due to shape anisotropy effects because of its rectangular shape, exchange coupling and surface roughness coupling ("orange-peel coupling") which act to provide the effective easy axis along the length of the vertical sensor. Changing the thickness of the film will change the surface roughness coupling to a greater degree than it will change the shape anisotropy to provide some control of the result, and the composition of the ferromagnetic layers can be changed to lower the anisotropy field across the sensing structure width to provide further control. The magnitude of the resulting lengthwise effective anisotropy field is significantly less than that which would be provided if the fabricated easy axis intentionally provided during fabrication along the length of the vertical sensing structure.
The value of having an effective anisotropy field of a reduced value directed along the length of the vertical sensor can be seen from a further approximation for the magnetic torque set out in the above torque equation by neglecting the curling of the magnetic field at the long edges of the vertical sensor and neglecting the demagnetizing fields. As a result, the equation represents approximately the sensing situation occurring near the center of the width of the vertical sensor relatively far from the edges, and represents that as though this condition applied over the entire width of the sensor. In addition, the bias field can be ignored for this purpose. The result of such an approximation is EQU M.sub.s H.sub.sg cos .theta.-M.sub.s H.sub.sn sin .theta..+-.M.sub.s H.sub.k sin .theta.cos .theta.=0.
An alternative algebraic sign indication, .+-., has been placed ahead of the anisotropy term to indicate that the anisotropy field term leads to opposite polarity torques depending on whether the easy axis with respect to which the field occurs is, for two possibilities, either across the width of the sensing structure or along the length of the sensing structure. As indicated, the first magnetic torque equation above was written assuming that the effective direction of the easy axis was across the sensor.
Since the response of the sensing structure to an externally applied signal field, i.e. the effective permeability, as indicated above is proportional to the sine of .theta., this last equation can be solved for the sine of .theta. to get an expression related to the effective permeability or ##EQU3## In this last solution, the precedence of the denominator term H.sub.k with a plus sign corresponds to the easy access being across the width of the stripe. In this circumstance, one can see that an increase in the sense field H.sub.sn will in this circumstance only serve to reduce the effective permeability and hence provide little aid in increasing the output signal in response to a externally applied signal field H.sub.sg.
In the opposite situation in which a minus sign precedes the anisotropy field H.sub.k in the last equation for a lengthwise easy axis, however, a relatively high effective permeability can be maintained even with large sense fields through adjusting the anisotropy field value to be relatively near in magnitude to the desired sense field magnitude. Thus, if the difference in magnitudes between sense field H.sub.sn, acting across the width of the vertical sensor in response to the sense current traversing the length of that sensor, and anisotropy field H.sub.k is kept small through adjustment of the anisotropy field value by appropriate adjustment of the combined exchange coupling, "orange-peel" coupling, and shape anisotropies, a relatively large effective permeability can be provided in the vertical sensing structure.
As indicated above, the resistance change along the length of the vertical sensor is proportional to the sign of the angle between the magnetizations in the two ferromagnetic layers on either side of the intermediate layer. The output signal is formed from the voltage changes occurring across the length of the sensing structure for a fixed sense current therethrough because of the magnetization direction transitions in the magnetic media passing by the sensor that act to change the angle between the magnetization vectors in the two ferromagnetic layers, an angular change which is related to the effective permeability as described above. Thus, the magnitude of the output signal will depend on the amount of resistive change in the sensor which in turn will depend on the fraction of the length of that sensor which is penetrated by the externally applied signal field H.sub.sg causing such angular changes.
The degree that the externally applied magnetic field is shunted away from the vertical sensor by the high permeability shielding poles on either side thereof in a typical "read" sensing head determines the effective fraction of, or the effective length in, the vertical sensing structure that is subject to this externally applied field. The effective signal field in the vertical sensing structure can be found from an Ampere's law line integral through the entire field over a closed path through the length of the vertical sensing structure and across the gaps and then through the length of a shielding pole positioned adjacent to the vertical sensor across the gap therefrom. Such a procedure can, through approximations, be used to arrive at a nonuniform transmission line model equation for the average signal flux distribution in the vertical sensing structure due to a magnetization transition in the magnetic media moving pass the sensor which is modeled as a flux source. The average signal flux in the vertical sensing structure is represented by M.sub.av which is averaged across the width of the sensor to provide the average signal flux at any point along the length of that sensor. The result is ##EQU4## Here, T again is the thickness of a magnetic layer, .mu..sub.r is the average effective permeability, G.sub.ss (z) is the gap distance between the vertical sensing structure and one of the shielding poles, and M.sub.b is the bias flux resulting from a bias current in the bias strap provided adjacent to the vertical sensing structure. The variable "z" is the directional coordinate along the length of the sensor intersecting the "x" axis at its zero datum.
An analytical solution can be obtained to this equation with the simplifying assumption that the gap distance between the vertical sensor and the shielding poles on either side thereof is constant with a value of G.sub.ss-c giving a total gap for the gaps on both sides of the sensor of G=2G.sub.ss-c. Although this is not often the case in practice, such an approximation allows obtaining a representative distance expression for the length of the vertical sensor which is effectively penetrated by the flux due to the externally applied signal field changes caused by the magnetization transitions in the magnetic media passing by the end of the vertical sensing structure. Such an approximation, along with ignoring the bias field magnetization in view of interest being only in the externally applied signal field decay with length in the sensing structure, yields ##EQU5##
This equation has an exponential solution for M.sub.av involving the base e raised to an exponent involving -z divided by a characteristic length. This characteristic length is found to be ##EQU6## However, there are two shielding poles present in practice separated from the vertical sensor by two gaps, but also there are present two magnetic layers in the "sandwich" structure of that sensor. Thus, this characteristic length in practice will remain the same as that just given to thereby provide an effective decay length, d, in the vertical sensing structure measured from the air bearing surface at which the externally applied signal field is down by 1/e from its initial value, this length being ##EQU7##
Thus, the output voltage signal will depend on the effective permeability in the vertical sensor, the thickness of the magnetic layers therein and the length of the gap. There is a desire to increase the magnitude of the output signal for a given value of externally applied signal H.sub.sg provided by magnetization transitions in the magnetic media passing by the end of the vertical sensor, as such externally applied signals tend to become smaller as less and less magnetic material is provided for each bit in each data track because of a desire to increase the density of storage in the magnetic media. Thus, there is desired a sensor configuration which can yield an increased output signal for a given externally applied input signal without resulting in widening the vertical sensor which would limit the narrowness permitted for tracks in the magnetic media.